Alfred, thanks. We had clocked this link before, but despite these instructions we've still failed - which may be a sign of our technical ineptitude, or that the problems with our cap are more serious.
You're right to say that our car has the earlier three-key setup - although right now we're working with four, since the black plastic filler cap on the car (being non-original) has its own separate key, rather than being coded to one of the car's original three. If we were to find a replacement metal cap it would need to come with a key, since I wouldn't expect to find a cap which matched one of the car's factory keys ... ... ... but given that we're already used to working with four keys that wouldn't be an issue.
Martin